The present invention relates to the field of ion chromatography (IC), and, in particular, to apparatus and methods of ion chromatography wherein gas is removed prior to detection of sample ions.
Suppressed ion chromatography (SIC) is a commonly practiced method of ion chromatography which generally uses two ion-exchange columns in series followed by a flow through conductivity detector for detecting sample ions. The first column, called the analytical, chromatography or separation column, separates the analyte ions (e.g., the sample ions) in a sample by elution of the analyte ions through the column. The analyte ions are flowed through the analytical column via a mobile phase comprising electrolyte. Generally, a dilute acid or base in deionized water is used as the mobile phase. From the analytical column, the separated analyte ions and mobile phase are then flowed to the second column, which is called the suppressor or stripper. The suppressor serves two primary purposes: (1) it lowers the background conductance of the mobile phase by retaining (e.g., suppressing) the electrolyte of the mobile phase, and (2) it enhances the conductance of the analyte ions by converting the analyte ions to their relatively more conductive acid (in anion analysis) or base (in cation analysis). The combination of these two functions enhances the signal to noise ratio, and, thus, improves the detection of the analyte ions in the detector. Accordingly, upon exiting the suppressor, the analyte ions and suppressed mobile phase are then flowed to the detector for detection of the analyte ions. A variety of different types of suppressor devices and methods are discussed in U.S. Pat. Nos. 3,897,213; 3,920,397; 3,925,019; 3,926,559; and U.S. Ser. No. 08/911,847. Applicants hereby incorporate by reference the entire disclosure of these patent applications and patents.
As those skilled in the art will appreciate, both the mobile phase and the sample contain counterions of the analyte ions. A suppressor operates by ion exchange of suppressor ions, which are located in the suppressor, with both (1) the mobile phase electrolyte counterions and (2) the sample counterions. In anion analysis, for example, the suppressor ions normally comprise hydronium ions and the mobile phase comprises electrolyte such as sodium hydroxide or mixtures of sodium carbonate and sodium bicarbonate. In cation analysis, the suppressor ions normally comprise hydroxide ions, and the mobile phase may comprise electrolytes such as hydrochloric acid or methanesulfonic acid. The suppressor ions are located on a stationary phase, which may be an ion exchange membrane or resin or both. As the mobile phase and sample (which contains both analyte ions and counterions of the analyte ions) are flowed through the stationary phase of the suppressor, the electrolyte counterions in the mobile phase and the sample counterions are retained on the stationary phase by ion exchange with the suppressor ions. When the suppressor ions are either hydronium or hydroxide, ion exchange of the electrolyte counterions with suppressor ions converts the mobile phase to water or carbonic acid, which are relatively non-conductive. On the other hand, the ion exchange of sample counterions with suppressor ions (i.e., hydronium or hydroxide ions) converts the analyte ions to their relatively more conductive acid (in anion analysis) or base (in cation analysis). Thus, the analyte ions, which are now in their relatively more conductive acid or base form, are more sensitive to detection against the less conductive background of the mobile phase.
However, unless the suppressor ions are continuously replenished during the suppression process, the concentration of suppressor ions on the stationary phase is reduced. Eventually the suppressor will become exhausted and its suppression capacity is either lost completely or significantly reduced. Thus, the suppressor must be either replaced or regenerated. The need to replace or regenerate the suppressor is inconvenient, may require an interruption in sample analysis, or require complex valving or regeneration techniques known in the art. Methods of electrochemically regenerating an at least partially exhausted suppressor are known in the art. See, for example, U.S. Pat. Nos. 5,633,171 and 5,773,615, which are directed to intermittent electrolytic packed bed suppressors. The assignee of this application also discloses, among other things, similar methods of intermittent electrochemical regenerating of a suppressor in U.S. Pat. No. 5,759,405. A method of an intermittent, but xe2x80x9cfrequent,xe2x80x9d chemical regeneration of a suppressor is disclosed in U.S. Pat. No. 5,597,734. One problem associated with such xe2x80x9cintermittentxe2x80x9d methods of electrochemically regenerating a suppressor is that the suppressor being regenerated must be taken xe2x80x9coff-linexe2x80x9d, that is, while being regenerated the suppressor is not used in a sample or analysis run. An example of a known technique for continuously regenerating a suppressor by continuously replenishing suppressor ions is disclosed in U.S. Pat. No. 5,352,360.
Another problem associated with SIC is that a separate suppressor unit is usually required, and, therefore, the number of components in the system is increased over traditional IC systems. Traditional IC systems usually contain a mobile phase source, a pump, a sample injector, an analytical column and a detector for detecting the sample ions. In SIC, a separate suppressor unit is added to the system. This, in turn, increases the complexity of the system and also increases extra-column volume which may decrease chromatographic resolution and sensitivity. Therefore, it would be advantageous to have a system of ion suppression chromatography which reduced the number of system components in traditional SIC systems.
Another problem associated with prior art SIC systems is that the mobile phase is converted to a weakly ionized form, which renders the mobile phase unsuitable for reuse. Thus, it would be advantageous if a system of SIC were developed in which the mobile phase is converted back to its strongly ionized form after suppression and, thus, may be reused.
Another problem associated with SIC systems using sodium carbonate/bicarbonate mobile phases is that suppression of the mobile phase yields carbonic acid which interferes with the detection of the sample ions. More specifically, when a sodium carbonate/bicarbonate eluant is used, during suppression of the sodium electrolyte carbonic acid is formed. The carbonic acid is more conductive than water and creates xe2x80x9cbackground noisexe2x80x9d which interferes with detection of the sample ions.
In its various aspects, the present invention addresses one or more of the foregoing problems associated with SIC.
In one aspect of the invention, a method of continuous electrochemically suppressed ion chromatography is provided. Analyte ions in a mobile phase comprising electrolyte are separated in a chromatography column resulting in a chromatography effluent comprising electrolyte and separated analyte ions. The chromatography effluent is then split into a first chromatography effluent stream and a second chromatography effluent stream. Electrolysis ions selected from the group consisting of hydronium ions and hydroxide ions are generated by the electrolysis of water. The electrolysis ions having the same charge as the electrolyte and the second chromatography effluent stream, which contains electrolyte and analyte ions, are simultaneously flowed through a stationary phase thereby suppressing the electrolyte in the second chromatography effluent stream. In a preferred aspect of the invention, the electrolysis ions force the electrolyte away from the second chromatography effluent stream and into the first chromatography effluent stream thereby effectively suppressing the second chromatography effluent stream. The analyte ions in the suppressed second chromatography effluent stream are then detected.
In another aspect of the invention, a suppressor adapted for use in a method of continuous electrochemically suppressed ion chromatography is provided. The suppressor comprises an inlet, a first outlet, a second outlet and a third outlet. A first stationary phase comprising ion exchange resin is positioned in the path of fluid flow through the suppressor from the inlet to the third outlet. A second stationary phase comprising ion exchange resin is positioned in the path of fluid flow through the suppressor from the inlet to the first outlet. A first regeneration electrode is positioned at the third outlet and a second regeneration electrode is positioned at the second outlet.
In yet another aspect of the invention, the suppressor further comprises sensor electrodes positioned in the second stationary phase for detecting the analyte ions in the suppressor.
In yet another aspect of the invention, a method of suppressed ion chromatography is provided wherein the suppressed chromatography effluent is converted back to its strongly ionized state after suppression. Thus, the mobile phase is recycled and may be reused in a subsequent sample run.
In a further aspect of the invention, a method of continuous electrochemically suppressed ion chromatography is provided where analytical column effluent, which contains separated analyte ions and electrolyte, is flowed to a first inlet of a suppressor. The suppressor comprises a stationary phase. The chromatography effluent is flowed through at least a portion of the stationary phase to suppress the chromatography effluent. The suppressed chromatography effluent is flowed to a detector where the analyte ions are detected. The detector effluent is then flowed back to the suppressor through a second inlet and out a second outlet to waste.
In another aspect of the invention, a suppressor is provided wherein the same suppressor may be used in both anion and cation analysis. The suppressor has a first inlet, a first outlet and a second outlet. A first stationary phase is located in the path of fluid flow through the suppressor from the first inlet to the first outlet. A second stationary phase is located in the path of fluid flow through the suppressor from the first inlet to the second outlet. A pair of regeneration electrodes are further provided wherein the first and second stationary phases are located between the electrodes such that an electrical potential may be applied across the first and second stationary phases. The first and second stationary phases further comprise oppositely-charged ion exchange resin.
In yet a further aspect of the invention a method of SIC is provided using a sodium carbonate/bicarbonate mobile phase. Analyte ions in a mobile phase comprising sodium carbonate/bicarbonates are chromatographically separated to yield a chromatography effluent comprising separated analyte ions and a sodium carbonate/bicarbonate mobile phase. The sodium carbonate/bicarbonate mobile phase is then suppressed to yield a suppressed chromatography effluent comprising carbonic acid, carbon dioxide gas and separated analyte ions. Prior to detecting the analyte ions, the carbon dioxide gas is removed.
In yet another aspect of the invention, analyte ions in an aqueous mobile phase comprising electrolyte are chromatographically separated to form an aqueous chromatography effluent comprising separated analyte ions and electrolyte. The electrolyte is suppressed by ion exchange with electrolysis ion selected from the group consisting of hydronium ions and hydroxide ions which are generated by the electrolysis of water; the electrolysis of the water further forming gas by-products. The gas by-products are then removed from the separated analyte ions and the separated analyte ions are subsequently detected.